Method and apparatus for assessing cardiac funtional status

ABSTRACT

A method for assessing cardiac function suitable for incorporation into an implantable cardiac rhythm management device. By measuring daily exertion levels in accordance with the invention, an assessment of cardiac function can be made that has been found to correlate well with conventional clinical classifications. The invention also provides for assessing cardiac function in conjunction with different pacing schemes designed to treat heart failure and using the assessment to select the best such scheme for the patient.

FIELD OF THE INVENTION

[0001] This invention pertains to systems and methods for deliveringpacing and other therapies to treat cardiac conditions and for assessingthe effectiveness of such therapies.

BACKGROUND

[0002] Congestive heart failure (CHF) is a clinical syndrome in which anabnormality of cardiac function causes cardiac output to fall below alevel adequate to meet the metabolic demand of peripheral tissues. CHFcan be due to a variety of etiologies with that due to ischemic heartdisease being the most common. Symptoms of CHF in certain instances canbe due to cardiac arrhythmias that are treatable with conventionalbradycardia pacing. Some CHF patients suffer from some degree of AVblock such that their cardiac output can be improved by synchronizingatrial and ventricular contractions with dual-chamber pacing (i.e.,pacing both the atria and the ventricles) using a short programmed AVdelay time. It has also been shown that some CHF patients suffer fromintraventricular conduction defects (a.k.a. bundle branch blocks) suchthat their cardiac outputs can be increased by improving thesynchronization of right and left ventricular contractions. Cardiacpacemakers have therefore been developed which provide pacing to bothventricles, termed biventricular pacing.

[0003] In CHF patients who are treated with pacing therapy (e.g., eithera conventional pacemaker or a biventricular pacemaker), it is desirableto select a pacing scheme that optimally improves the patient'scondition. Examples of a pacing scheme include a particular pacing modeand parameter values related to that mode such as lower rate limit, AVdelay time, and biventricular delay time. Pacing schemes areconventionally selected based upon a clinical assessment of thepatient's condition. For example, EKG data may indicate a patient islikely to be benefited more with biventricular pacing than withconventional dual-chamber pacing. After being set initially, the pacingscheme can then be adjusted on a trial and error basis to a more optimumone based upon the patient's history and physical examination insubsequent office visits. This also allows the pacing scheme to beadjusted in accordance with any changes that occur in the patient'sphysical condition. This method of assessing a CHF patient's cardiacfunctional status can be a very subjective one, however, depending onthe physician's perception of the patient's symptoms and physicaldisability. There is a need, therefore, for a method of assessing a CHFpatient's functional status that is more accurate and reproducible thanthose currently practiced in order to select a pacing scheme. It istoward this objective that the present invention is primarily directed.

SUMMARY OF THE INVENTION

[0004] The present invention relates to a method for assessing thefunctional status of congestive heart failure patients that isparticularly suitable for use in selecting optimal pacing schemes forthose patients receiving pacing therapy. In accordance with theinvention, maximal exertion levels of the patient are tracked with animplantable device while the patient goes about his or her dailyactivities. Such maximal exertion levels have been found to correlatewell with other methods of classifying a patient's cardiac functionalstatus. In one embodiment, the method is performed by measuring a movingaverage over a specified averaging period of exertion levels attained bythe patient during daily activities. Daily maximal exertion levels arethen extracted from the measured moving average exertion levels for aspecified number of days, and the patient's cardiac functional status isclassified based upon the daily maximum daily exertion levels. Theexertion levels are measured by a sensor that measures a physiologicalvariable related to exertion level such as an accelerometer or minuteventilation sensor.

[0005] The method may be incorporated in a cardiac pacemaker used totreat congestive heart failure where the functional status assessment isused to either automatically select an optimum pacing scheme or to aidthe clinician in making the selection. In such a device, a processor forcontrolling the operation of the device is programmed to perform themethod using data collected from an exertion level sensor. The dailymaximal exertion levels are registered and stored by the device, and maythen be transmitted to an external programmer. The processor may also beprogrammed to automatically adjust a pacing scheme of the device basedupon the extracted maximal exertion levels obtained during a testingsequence in which different pacing schemes are tried.

BRIEF DESCRIPTION OF THE DRAWINGS

[0006]FIG. 1 is a system diagram of a pacemaker.

[0007]FIG. 2 is a flow chart illustrating the steps of an exemplarytesting sequence.

DESCRIPTION OF SPECIFIC EMBODIMENTS

[0008] Assessing the functional status of a heart failure patient isusually done with a comprehensive history and physical examination. Thephysician's perception of the patient's symptoms and physical disabilityis then used to classify the patient's status according to aclassification system such as that put forth by the Criteria Committeeof the New York Heart Association (NYHA). It has been found thatmeasured maximum exertion levels of a patient during daily living arewell correlated to such classifications. In accordance with theinvention, a patient's cardiac functional status is classified basedupon measurements of maximal exertion levels attained by the patient asthe patient goes about normal activities during the day. The exertionlevel measurements are performed by an implantable device such as apacemaker or a portable external device. In a particular embodiment,exertion level signals from, e.g., an accelerometer and/or minuteventilation sensor are automatically collected by an implantable deviceand moving averaged over a period from 30 seconds to 10 minutes, withthe daily maximum moving average value being registered and stored. Thedaily maximum moving average value is then averaged over a specifiedtime period (e.g., on a weekly, monthly, or semiannual basis) with theresulting average maximum exertion level also registered and stored. Bygathering exertion level data while different therapies are being tried,an indication of the effectiveness of the therapy in treating thepatient's condition can be obtained.

[0009] The preferred way of implementing the exertion level measurementsis to employ the same type of exertion level sensor used to controlrate-adaptive pacemakers. A rate-adaptive pacemaker varies the pacingrate in accordance with a measured physiological variable related tometabolic rate or exertion level. (See, e.g., U.S. Pat. No. 5,376,106issued to Stahmann et al., the disclosure of which is herebyincorporated by reference.) Measuring minute ventilation, which is theproduct of ventilation rate and tidal volume, estimates oxygenconsumption which is related to exertion level. Another way ofdetermining exertion level is by measuring body activity or motion witheither an accelerometer or vibration sensor. The activity-sensingpacemaker uses a microphone-like sensor (e.g., an accelerometer) insidethe pacemaker case that responds to mechanical vibrations of the body byproducing electrical signals proportional to the patient's level ofphysical activity.

[0010] The present invention can be used to assess the effectiveness ofany therapy used to treat congestive heart failure or otherwise improvea patient's cardiac functional status. In the embodiments describedbelow, the invention is incorporated into the control unit of animplantable pacemaker and used to evaluate different pacing schemes.Examples of such schemes include a particular bradycardia pacing mode, alower rate limit, parameters related to rate-adaptive pacing, aprogrammed atrio-ventricular delay for dual-chamber pacing, and aninterventricular delay for biventricular pacing. As noted above,biventricular pacemakers are sometimes used in the treatment ofcongestive heart failure where pacing pulses are delivered to bothventricles by separate electrodes during a cardiac cycle. (See, e.g.,U.S. Pat. Nos. 5,792,203 and 4,928,688 which are hereby incorporated byreference.) In a biventricular pacemaker, the right and left ventriclesare sensed through separate channels with either both ventricles or onlythe left ventricle being paced upon expiration of a selected lower rateinterval without receipt of a right ventricular sense signal, where thelower rate interval starts with either a pacing event or receipt of aright ventricular sense signal. In the case of biventricular pacing, theventricles may be paced simultaneously or one after the other separatedby a selected delay interval. The pacemaker may also be operated in aventricular triggered mode where one or both ventricles are paced withina latency period following a sense signal from the right ventricle. Suchdifferent pacing schemes may vary in their effectiveness for any givenpatient, and the present invention provides a way of ascertaining whichscheme is best.

[0011] Shown in FIG. 1 is a microprocessor-based pacemaker with thecapability of delivering conventional bradycardia pacing to the atriaand/or ventricles or biventricular pacing. (As used herein, the termpacemaker should be taken to mean any cardiac rhythm management devicewith a pacing functionality including an implantablecardioverter/defibrillator that includes a pacemaker). A microprocessor10 is the primary component of the device's control unit andcommunicates with a memory 12 via a bidirectional data bus 13. Thememory 12 typically comprises a ROM (read-only memory) for programstorage and a RAM (random-access memory) for data storage. The pacemakerhas atrial sensing and pacing channels comprising electrode 34, lead 33,sensing amplifier 31, pulse generator 32, and an atrial channelinterface 30 which communicates bidirectionally with a port ofmicroprocessor 10. The device also has ventricular sensing and pacingchannels for both ventricles comprising electrodes 24 a-b, leads 23 a-b,sensing amplifiers 21 a-b, pulse generators 22 a-b, and ventricularchannel interfaces 20 a-b where “a” designates one ventricular channeland “b” designates the other. For each channel, the same lead andelectrode are used for both sensing and pacing. The channel interfaces20 a-b and 30 include analog-to-digital converters for digitizingsensing signal inputs from the sensing amplifiers and registers whichcan be written to by the microprocessor in order to output pacingpulses, change the pacing pulse amplitude, and adjust the gain andthreshold values for the sensing amplifiers. A telemetry interface 40 isalso provided for communicating with an external programmer. An exertionlevel sensor 50 is provided to measure the patient's exertion levels inaccordance with the invention as well as provide the capability forrate-adaptive pacing. The exertion level sensor may be a sensor formeasuring a physiological variable related to the patient's level ofphysical exertion such as an accelerometer or a minute ventilationsensor.

[0012] Also shown interfaced to the microprocessor 10 are a number ofinterval timers 11 to be discussed below. These timers may either bediscrete counters as shown or be implemented in software by themicroprocessor executing programmed instructions in memory 12. Apacemaker is a device which paces one or more chambers based upon sensedevents and the outputs of interval timers. In an atrial triggeredpacemaker, timers for the following intervals are provided: lower rateinterval (LRI) which defines the minimum rate at which the ventricleswill be paced in the absence of spontaneous activity, atrioventriculardelay interval (AVD) which defines the time delay between an atrialpulse or sense and the ensuing ventricular pace if no ventricular senseoccurs in the interval, and atrial escape interval (AEI) which defines aminimum rate for atrial pacing in the case of an atrial paced mode. Forbiventricular pacing, an interval timer for the interventricular delay(IVD) is provided that defines the delay between the time the twoventricles are paced. The IVD, for example, can be set to zero to enablesimultaneous pacing of the ventricles or can be set to a positive ornegative value to enable sequential pacing of the two ventricles in thespecified order and after the specified delay interval. Other intervaltimers are used to define refractory periods for the sensing channelsduring which time the channels are closed so that inputs are ignored.

[0013] In a particular embodiment of the invention, the method forassessing a patient's cardiac function as described above is implementedin the control unit of a pacemaker as illustrated in FIG. 1. In thisembodiment, the effectiveness of different pacing schemes is tested byassessing the patient's cardiac function as different pacing schemes aretried. The test data is then employed to either automatically adjust thepacing scheme or is stored for later retrieval by a clinician. Forexample, a pacemaker with biventricular pacing capability might testfour different pacing schemes on a CHF patient: left ventricular pacingwith an AVD equal to 100 ms, left ventricular pacing with an AVD of 50ms, biventricular pacing with an IVD of 0 and an AVD of 120 ms, andbiventricular pacing with an IVD of 20 ms and an AVD of 180 ms. Each ofthese pacing schemes could be tried for a specified period (e.g., 30days) with their effectiveness assessed by their effect on the patient'scardiac functional status as determined by the maximal exertion levelmethod. In another example, a rate-adaptive bradycardia pacemaker havingboth an accelerometer and a minute ventilation sensor for measuringexertion levels might test three rate-adaptive pacing schemes: using theminute ventilation sensor only, using the accelerometer only, and usingboth sensors. A rate-adaptive pacemaker could also test different slopefactors.

[0014]FIG. 2 shows a flowchart of an exemplary version of the testingsteps according to the present invention. The method starts (step S0) byperforming any necessary initialization of variables. Various parametersused by the method are then set in accordance with preprogrammed valuesthat may be modified by an external programmer (step S1): the number ofwash out days (WO_DAYS) during which no therapy is delivered whilecardiac function is assessed, the number of test days (TEST_DAYS) forwhich maximal exertion levels are to be determined, the number oftherapies (e.g., pacing schemes) that are to be tested (#_THRPY),whether the pacing scheme is to be adjusted automatically in aclosed-loop manner or not (AUTO), and the default pacing scheme (DFPS)that the device defaults to initially and reverts to if the AUTO switchis not in automatic mode. The averaging period AVG_P is then set (stepS2), which is the period over which exertion levels are moving averagedbefore extraction of a maximal level as described above. The order ofthe therapies that are to be tested is then randomized, and a therapylist is created (step S3). The next therapy in the list is thenprogrammed into the device (step S4). In this example, a daily maximalexertion level is determined, and the daily levels are then averagedover the total number of test days. A 24 hour timer is reset (step S5),and maximal exertion levels are then extracted from moving average datataken during the day. The maximal exertion levels are measured over theaveraging period (step S6), and the step iterates until 24 hours haveelapsed (step S8). Daily maximal exertion levels determined for thespecified number of test days (step S9) which could be, for example, aweek or a month. If so, an average daily maximal exertion level over thetotal number of test days is computed (step S10). The device is thenconfigured to deliver no pacing therapy for a specified number of washout days before the therapy is switched to the next one on the therapylist (step S11). It is next determined if all therapies on the list havebeen tested (step S12), and the next therapy on the list is tested (stepS4) if not. Otherwise, the exertion level data for all of the therapiesare stored (step S13). Depending on whether the automatic mode isenabled or not (step S14), either the therapy resulting in the bestfunctional status for patient is selected for use by the device (stepS15) or the device reverts to the default therapy (step S17), and themethod is ended (step S16). Whether or not the automatic mode isenabled, the stored exertion level data is available for downloading toan external programmer and analysis by a clinician.

[0015] If the automatic mode is not enabled, the exertion levelinformation associated with different therapies is displayed to theclinician upon interrogation of the device and can be used to select thebest therapy. The stored exertion level data also provides a long-termhistory of maximal exertion levels that are representative of apatient's capability to perform physical activities and general state ofhealth. Such a history may be useful to the clinician in makingtreatment decisions or in diagnosis.

[0016] Therapies other than pacing schemes may also be tested andevaluated by the method described above in which exertion levels aremeasured while different therapies are tried. This may involveselectively turning on or turning off different device therapeuticfeatures to provide optimum therapy or reconfiguring parameter settings.For example, an implantable drug delivery device capable of deliveringdifferent drug therapies to treat conditions affecting a patient'scardiac functional status may use the present method to select the besttherapy. A device can also be programmed to activate an additionaldiagnostic monitoring operation when there is a change in the patient'sstatus as reflected by measured exertion levels. Such diagnosticmonitoring may be normally turned for resource conservation or becausethe patient is required to exercise in order for the test to beperformed. For example, rate dependent bundle branch block can bedetected by activating AV delay monitoring when the exertion level isabove a certain threshold.

[0017] Although the invention has been described in conjunction with theforegoing specific embodiment, many alternatives, variations, andmodifications will be apparent to those of ordinary skill in the art.Such alternatives, variations, and modifications are intended to fallwithin the scope of the following appended claims.

What is claimed is:
 1. A method for assessing a patient's cardiacfunctional status, comprising: measuring a moving average over aspecified averaging period of exertion levels attained by the patientduring daily activities; extracting daily maximal exertion levels fromthe measured moving average exertion levels for a specified number ofdays; and, classifying the patient's cardiac functional status basedupon the daily maximum daily exertion levels.
 2. The method of claim 1wherein the exertion levels are moving averaged over a time period ofbetween approximately 30 seconds and 10 minutes.
 3. The method of claim1 wherein the exertion levels are measured by an implantableaccelerometer.
 4. The method of claim 1 wherein the exertion levels aremeasured by an implantable minute ventilation sensor.
 5. The method ofclaim 1 wherein the steps are performed automatically at periodic timesby an implantable device.
 6. The method of claim 5 wherein the dailymaximal exertion levels are registered and stored by the device.
 7. Themethod of claim 6 further comprising transmitting the registered andstored daily maximal exertion levels to an external programmer for thedevice.
 8. The method of claim 1 further comprising computing andstoring averages of daily maximal exertion levels over a specifiedperiod of time.
 9. The method of claim 1 further comprisingautomatically adjusting a pacing scheme of the device based upon theextracted maximal exertion levels.
 10. The method of claim 5 furthercomprising automatically selecting a drug therapy capable of beingdelivered by the device.
 11. The method of claim 5 further comprisingactivating a selected diagnostic monitoring feature performed by thedevice when measured maximal exertion levels reach a selected threshold.12. The method of claim 5 further comprising turning off or turning ondevice therapeutic features in accordance with measured maximal exertionlevels.
 13. The method of claim 5 further comprising: performing a testsequence in which different therapies capable of being delivered by thedevice are tried while maximal exertion levels associated with thetherapy are measured; and, automatically reconfiguring device parametersettings to select the optimum therapy for delivery by the device inaccordance with the measured exertion levels.
 14. The method of claim 13wherein the device is a cardiac pacemaker and the different therapiesare pacing schemes.
 15. A cardiac rhythm management device, comprising:sensing channels for sensing atrial and ventricular depolarizationsignals, each channel including an electrode and a sense amplifier; apulse generator for delivering pacing pulses to the atrium and/orventricle; an exertion level sensor; and, a processor for controllingthe delivery of pacing pulses in response to elapsed time intervals anddetected depolarization signals in accordance with a programmed mode,wherein the processor is programmed to perform the method set forth inclaim
 1. 16. The device of claim 15 wherein the processor is programmedto automatically adjust a pacing scheme of the device based upon theextracted maximal exertion levels.
 17. The device of claim 16 whereinthe processor is programmed to perform a testing sequence in whichdifferent pacing schemes are tried for predetermined periods and apatient's functional status is assessed while each scheme is tried. 18.The device of claim 17 wherein the processor is programmed to insert awash out period between trials of different pacing schemes.
 19. Thedevice of claim 17 wherein the processor is programmed to automaticallyselect for use the pacing scheme resulting in the most improvedfunctional status of a patient as determined by the testing sequence.20. The device of claim 15 wherein the processor is programmed toactivate a diagnostic monitoring operation when there is a change in thepatient's status as reflected by measured exertion levels.